F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Lecture 6Lecture 6

Shot Receiver

Layer 1

Layer 2

Layer 3

Layer 4

Impedance = Velocity * Density

Seismic Record

Travel T

ime (2 w

ay) in m

sec

0

+Peak

-Trough

Layer 1

Layer 2

Layer 3

Layer 4

Layer 2

Layer 3

ImpedanceIncrease

ImpedanceDecrease

ImpedanceIncrease

Peak over Trough is anIncrease in Impedance

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

The Ideal Seismic Response

Increase in Impedance Decrease in Impedance

Able to resolve boundaries of beds a few meters thick

1 meter

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Scale for Seismic Data

Although seismic data can not image small-scale stratal units, it can image mid- to large-scale units Parasequences

Bed Sets

Parasequence Sets

Sequences

Beds

Lamina Sets

Lamina

Sequence Sets

The big advantage of seismic data is areal coverage

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Seismic - Units 10s of Meters Thick

10 m

Predominantly

Shale

Predominantly

Shale

Predominantly

Sand

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Wave Equation Lingo

CompressionRarefaction

A A = Amplitude

λ = Wavelength length, ft or m

λ

P = Period time

Period = Time for the waveform to travel 1 wavelength

Dp = Pulse Duration time

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Basic Equations

1. P = 1 / f

2. λ = V * P = V / f

3. d = V * T / 2

where P = Period f = Frequency λ = Wavelength

V = Velocity d = distance (depth) T = time

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Back to Basics

Seismic energy travels down andis reflected off acoustic boundaries

Shot Receiver SeismicRecord

Increase inimpedance

Increase inimpedance

0.0

0.2

1.2

1.4

1.0

0.4

0.6

0.5

0.3

0.1

0.7

1.1

1.3

0.8

0.9

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Acoustic Structure of the Earth

Shot Receiver

I1 = 1 * V1

I2 = 2 * V2

I4 = 4 * V4

I3 = 3 * V3

ReflectionCoefficients

Pulse

CONVOLUTION

SeismicTrace

ImpedLow High

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

That ‘Pesky’ Pulse

TypicalPulse

SeismicTrace

ReflectionCoefficients

IdealPulse

SeismicTrace

If the frequency content (Bandwidth) is very large, then the pulse

approaches a spike and we can resolve fine-scale stratigraphy

Typically the frequency content is limited to about

10 to 50 Hz (BW = 40), which limits our resolution

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Types of Pulses

• Causal (real – no motion before wave arrives)

• Front loaded

• Peak arrival time is frequency dependant

• RC is at the first displacement; maximum displacement (peak or trough) is delayed by ¼ λ

ReflectionCoefficients

Minimum Phase

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Types of Pulses

• Not Causal (not real, since there is motion before the wave arrives)

• Symmetric about RC• Peak arrival time is not

frequency dependant• Maximum peak-to-side

lobe ratio• RC is at the maximum

displacement (peak or trough)

ReflectionCoefficients

Zero Phase

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Polarity – Minimum Phase

SEG Normal Convention

A compression is:• Negative # on the tape • Displayed as a Trough

ReflectionCoefficients

- +

SEG = Society of Exploration Geophysics

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Polarity – Zero Phase

SEG Normal Convention

A compression is:• Positive # on the tape • Displayed as a Peak

ReflectionCoefficients

- +

SEG = Society of Exploration Geophysics

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

What Causes Reflections?

• Any interface between bodies with different acoustic properties

• Acoustic properties define Impedance (I) , in which I = velocity * density

Shot Receiver

Layer 1

Layer 2 Boundary

Small change in impedance – small reflection

Large change in impedance – large reflection

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Time for Two Short Exercises

6a. Calculating Some Reflection Coefficients

6b. Calculating Frequency & Wavelength

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Seismic Interface

Shale

Sand

Velocity = 2000 m/sDensity = 1.7 gm/cc

Velocity = 2400 m/sDensity = 1.8 gm/cc

ReflectionCoefficient=

=

I below – I above

I below + I above

=

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Seismic Interface

Shale

Sand

Velocity = 2000 m/sDensity = 1.7 gm/ccI = 2000 * 1.7 = 3400

Velocity = 2400 m/sDensity = 1.8 gm/ccI = 2400 * 1.8 = 4320

ReflectionCoefficient=

4320 - 3400

4300 + 3400=

I below – I above

I below + I above

= 0.119

Of the incident energy, 12% is reflected, 88% is transmitted

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Seismic Interface

Shale

Carbonate

Velocity = 2000 m/sDensity = 1.7 gm/cc

Velocity = 2600 m/sDensity = 2.1 gm/cc

ReflectionCoefficient= =

I below – I above

I below + I above

=

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Seismic Interface

Shale

Carbonate

Velocity = 2000 m/sDensity = 1.7 gm/ccI = 2000 * 1.7 = 3400

Velocity = 2600 m/sDensity = 2.1 gm/ccI = 2600 * 2.1 = 5460

ReflectionCoefficient=

5460 - 3400

5460 + 3400=

I below – I above

I below + I above

= 0.232

Of the incident energy, 23% is reflected, 77% is transmitted

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Exercise 6b: Frequency & Wavelength

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Seismic Data & Stratal Surfaces

UnconformitiesStratal Surfaces

Facies Changes

Foreshore/Upper Shoreface

Submarine FanLower Shoreface - Offshore

Fluvial Incised Valley Fill

Estuarine

Coastal Plain

Slope - Basin

Condensed Interval

• Seismic reflections parallel stratal surfaces

• Reflection terminations mark unconformities

• Changes in reflection character indicate facies changes

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Why Stratal Surfaces?

Recall: Reflections are generated where there is a change in acoustic properties (I = v)

Brushy Canyon Formation, West Texas

Very Gradational LateralChanges in Physical Properties

Can Have Abrupt VerticalChanges in Physical Properties

Especially at PS Boundaries

Consider: Where can there be sharp changes in impedance?

• horizontally as lithofacies change? • vertically across stratal boundaries?

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Not Every Reflection is Strata!There are other seismic reflections out

there that may not be stratigraphic in origin

Stratal Surfaces

Facies Changes

• Fluid Contacts

• Fault Planes

• Multiples• Others

UnconformitiesO

W

G

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Another Exercise

6c. Generating a Modeled Seismic Trace

F W Schroeder

‘04

L 6 – Seismic Reflections

Courtesy of ExxonMobil

Exercise 6c: A Synthetic Trace

The Pulse

3 Ref. Coeff.